On-load tap changers (in abbreviation OLTC) are widely known and customary in the prior art. They serve for uninterrupted switching over between different winding taps of tapped transformers. On-load tap changers are divided into load selectors and load changeover switches.
Both kinds of on-load tap changers are actuated by a motor drive (preferably electric motor) for the switching over. A drive output or drive input shaft that draws up a drive element of the on-load tap changer, is moved by the motor drive. This drive element produces, for example, movement of a switching tube of the on-load tap changer and is usually termed force-storing unit (or also power store). When the force-storing unit is fully loaded, i.e. stressed, it is unlatched, abruptly releases its energy and actuates in a time period of milliseconds (ms) a switching tube that then executes a specific switching sequence during the load changeover. In that case, different switch contacts and resistance contacts are actuated in a specific time sequence. The switch contacts serve for direct connection of the respective winding tap with the load diverter and the resistance contacts for temporary connection, i.e. bridging over, by means of one or more switch-over resistances. Advantageously, vacuum interrupters are used as switch elements for the load changeover. This is based on the fact that the use of vacuum interrupters for the load changeover prevents arc formation in the oil and thus oil contamination of the load changeover switch oil, such as, for example, described in German Patent Specifications DE 195 10 809 [U.S. Pat. No. 5,834,717] and DE 40 11 019 [U.S. Pat. No. 5,107,200] and German published specifications DE 42 31 353 and DE 10 2007 004 5.
International Patent Application WO 1998/038661 discloses a motor drive for step switches, tap changers or plunger coils. The motor drive serves particularly for setting the step switch to the respectively desired switch setting. In the motor drive, all mechanical and electrical subassemblies required for drive of the step switch are combined together.
Based on that, a step switch of the reactor switch type is described in International Patent Application WO 2004/088693 [U.S. Pat. No. 7,463,010]. This similarly comprises a motor drive with downstream transmission, particularly a Geneva wheel transmission or lever deflecting transmission.
Chinese Utility Model CN 201242930Y discloses an on-load tap changer head for an on-load tap changer for uninterrupted switching over between different load step settings. The on-load tap changer head comprises an on-load tap changer transmission and a mechanical load step setting indicating means. For this purpose, an indicator shaft of the load step setting indicating means is directly coupled to a transmission shaft of the on-load tap changer transmission. The on-load tap changer head is provided with a cover with a viewing window through which the load step setting indicating means is readable. In particular, end setting mechanisms are provided so that the on-load tap changer cannot be connected beyond a start setting or end setting.
Chinese Utility Model CN 201167023Y discloses an on-load tap changer transmission with an indicating mechanism for switch-setting indicating means of an on-load tap changer. The on-load tap changer transmission comprises a transmission shaft with which a motor driving the switching tube of the on-load tap changer can be coupled. The transmission shaft drives a drive input shaft of the on-load tap changer transmission by a worm gear. A coupling element by which the drive input shaft is coupled with the switching tube of the on-load tap changer is formed at the lower side of an end of the drive input shaft remote from the motor. For indication of the switching position, an end of the indicating shaft projects from the housing of the on-load tap changer and is provided with a pointer rotatable relative to an indicating disk with switch setting marks. In order to improve readability, the pointer can be optically magnified.
A disadvantage of the on-load tap changer transmission with an indicating mechanism of Chinese Utility Model Specification CN 201167023Y is that the indicating mechanism records the movement of the drive input shaft for loading a force-storing unit. Derivation at the drive input shaft is susceptible to error, since possible slip between drive input shaft and force-storing unit is not detected, so that the indication of the switching position is imprecise.